As silicon technology advances to ultra-large scale integration (ULSI), the devices on Si wafers shrink to sub-micron dimension and the circuit density increases to several million transistors per die. In order to accomplish this high device packing density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines and the surface geometry such as corners and edges, of various features.
The requirement of small feature sizes with close spacing between adjacent features requires high resolution photolithographic processes. A problem that occurs during photolithography of a semiconductor wafer is caused by the reflectivity of a layer to be photopatterned. High reflectivity of aluminum alloy layers, for instance, causes degradation to occur in photoresist images through reflective light scattering.
In the past, one solution to this problem has been by the use of anti-reflective coatings (ARC). A film of amorphous silicon, for example, can be applied as an antireflection layer on top of a silicon substrate. Another example is the use of Ti and TiN over AlSi. These methods while useful also present certain process limitations. In particular, the use of an antireflective coating requires additional process steps. The antireflective coating must be accurately deposited in an optimal thickness and must be removed after photolithography. Moreover, some antireflective coatings may cause photoresist adhesion problems.
The present invention is directed to a process for increasing the optical absorptivity and decreasing the optical reflectivity of a metal layer in order to avoid reflective notching during photolithography.